Process for the filtration and combustion of carbonaceous matter emerging from internal combustion engines

ABSTRACT

The object of the invention is a filtration and combustion process for carbon particulate matter from an internal combustion engine. The process is characterized by the following steps: a rare earth or rare earth mixture derivative is introduced into the fuel at a concentration of 10 ppm to 500 ppm (by weight), and preferably of 20 ppm to 200 ppm; the soot produced by the internal combustion engine is collected on a filter, the temperature of the gases entering the filter being selected in the range of 100° C. to 350° C.; the soot is allowed to build up until a significant fraction or rate of incoming soot is balanced by the combustion of soot in the soot cake on the filter, and no is effected as long as the head loss caused by the soot does not exceed a preselected value and is not higher than 400 millibars. This invention is useful for organic synthesis.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The subject of the present invention is a process for the filtration andcombustion of carbonaceous matter emerging from internal combustionengines. The invention more particularly relates to the regulation ofthe pressure drop occasioned in filters by the accumulation of soot inthe filters.

2. Description of the Prior Art

During the combustion of motor fuels in internal combustion engines andespecially during that of diesel oil in diesel engines, carbonaceousmatter, hereinafter known as "soot", is formed which is supposed to beharmful both to the health of higher mammals and to the environment.

This soot is particularly abundant in the case of diesel engines andconstitutes a handicap for this type of engine. The majority of thesolutions envisaged consisting in changing the speed of the engine orits operating parameters come up against another constraint, that of notincreasing the emission of carbon monoxide and/or of gases which aresupposed to be toxic and mutagenic, such as nitrogen oxides.

Taking into account the above, the most effective technique seems to beto fit the exhaust silencer with a filter which is capable of stoppingall, or at the very least most of, the soot formed by the combustion ofthe various fuels.

Success has thus been achieved in producing filters, especially made ofcordierite, which make it possible to reduce soot emissions by at least85% by mass.

The problem to be solved lies in the accumulation of this soot in thefilters which causes, in the first place, an increase in the pressuredrop and, in the second place, blocking to begin which leads to a fallin efficiency of the internal combustion engine.

Many a time have combustion techniques of the above soot been tested. Ithas thus been proposed to cause combustion of this soot intermittentlyeither by electric heating or by heating by means of a fossil igniterfuel. The possibility has also been envisaged of drawing off the heatnecessary for igniting this soot from the engine itself, by skillfulmanagement of the gas flows so as to heat the soot accumulated in thefilter and ipso facto to cause its ignition (temperature of the order of500°-600° C).

It has also been proposed to introduce ignition catalyst precursors intothe various motor fuels, so as to lower the ignition temperature of thesoot. These techniques, especially that of combining increasing thetemperature of the soot using a suitable and transitory circuit of theexhaust gases with the addition of oxidation catalyst precursors, havemade it possible at least partially to solve the problem.

However, on the one hand, the ignition temperature of the soot remainsrelatively high (of the order of 500° C.) and, on the other hand, thereis the risk of the intermittent and violent combustions causing asignificant detrimental change in the filter and in its filteringabilities, either by cracking due to thermal shock or even by melting.

This detrimental change in the filters can take the form of a loss inability to retain a high percentage of soot, while the initialpercentage is sometimes considered to be insufficient.

SUMMARY OF THE INVENTION

For the above reason one of the aims of the present invention is toprovide a combustion process of the soot which is as continuous aspossible.

Another aim of the present invention is to provide a combustion processof the soot which does not produce violent, fierce and sudden ignitionsleading to damaging of the filter. In order to achieve such a result, itis advisable to prevent, during the ignition phase, the temperature fromreaching 1000° C., advantageously 900° C. and preferably 700° C. in anypart of the filter.

As it is difficult to measure the local temperatures, it was laid downthat these latter restrictions corresponded to maximum gas temperatures,at the outlet of the filter, of not more than approximately 600° C.("approximately" in this instance means that zeros, which are in thisinstance positional zeros, are not significant figures), preferably toapproximately 500° C.

Another aim of the present invention is to provide a process for thefiltration and combustion of soot which makes it possible to improve theproportion of soot retained on the filters and then subsequently burnt.

DETAILED DESCRIPTION OF BEST MODE AND PREFERRED EMBODIMENT OF THEINVENTION

These aims, and others which will subsequently appear, are achieved bymeans of a process for the treatment of soot containing one or a numberof rare-earth metals, in which process the said soot is brought intocontact with a gas containing oxygen at a temperature of between 100° C.and 400° C., advantageously between 150° C. and 350° C. and preferablybetween 200° C and 300°C., the oxygen partial pressure of the gascontaining oxygen being at least equal to 3% of atmosphere, i.e. 3×10³pascals, and advantageously to 4% of atmosphere, i.e. 4×10³ pascals.

It is even desirable for the oxygen partial pressure to be at leastequal to 6×10³ pascals and preferably to 8 kilopascals.

It is also desirable for the contact between the gas containing oxygenand the soot to be maintained for a time interval which is sufficient toburn at least 90% of the soot.

Indeed, in a completely unforeseen way, it was found that rare-earthmetals and especially rare-earth metal oxides, especially those ofcerium, catalysed oxidation of carbonaceous matter at temperatures aslow as 100° C. (this value is in fact a rounded value and it has evenbeen possible to record an oxidation at temperatures of the order of 80°C.).

This phenomenon only occurs when the oxygen content of the gas issufficiently high. The threshold beyond which oxidation of the soottakes place depends, to a certain extent, on the value of the otherparameters. It increases when the rare-earth metal content of the sootdecreases and/or when the temperature approaches values towards thelower end of the ranges mentioned above.

It should be noted that, during the preceding tests, this regenerationcould not be observed because the oxygen content of the exhaust gasesis, under the usual test conditions for such a process, chosen so as tomaximize the soot and is much less than that required for thisoxidation, which can be described as gentle.

The process according to the invention can be used in many ways forreducing emissions of solid carbonaceous discharge from all sootycombustions and especially from internal combustion engines.

It is thus possible to distinguish continuous operations from thosewhich are intermittent.

In the latter case, if the oxidation of the carbonaceous matter takesplace as soon as the combined soot and gas is at the appropriatetemperature, it is preferable for the oxygen partial pressure andtemperature to be maintained for a sufficiently long period of time inorder to oxidize all the soot produced in order to prevent the latterfrom accumulating or from being discharged. Thus, on average orstatistically or continuously, it is advisable that, for a definedperiod of time, the sum of the time intervals where the conditions ofthe present invention are combined is sufficient for the soot producedduring the defined period of time to be completely oxidized.

The restriction imposed on the system for producing and purifying sootdepends on the said defined period of time. In the case where, as inparticle filters, the residence time is high, or even infinite, withrespect to their life expectancy, it is possible to provide long definedperiods of time. The defined period of time can then reach 10 minutes orindeed 1/2 hour.

In the cases of filter-equipped internal combustion engines, a longdefined period of time is paid for by periods during which the pressuredrop occasioned by the filter can be relatively high. However, theaccumulation of particles in the filters improves the quality offiltration. There is therefore a compromise to be found.

Thus, it is therefore preferable to choose a defined period of timewhich prevents the carbonaceous particles from causing a pressure dropgreater than approximately half an atmosphere. More precisely, desirably2/5 atmosphere (i.e. 2/5×10⁵ pascals), advantageously of greater than1/4 of atmosphere (i.e. 1/4×10⁵ pascals), preferably of greater than 1/5atmosphere 0.2×10⁵ pascals and, if it is not desired to improve thefiltration characteristics of the soot, more preferentially 0.15atmosphere (i.e.0.15×10⁵ pascals.

According to an implementation of the present invention, it is possibleto vary the parameters of the engine and the optional introduction of agas containing oxygen so that the temperature and oxygen contentconstraints are fulfilled so that, during a so-called "European" cycle(EEC standard), there is no accumulation of soot in the filter.

It is also possible to subject the implementation of the processaccording to the present invention to a pressure drop limit value.

The time interval necessary for gentle oxidation of a given amount ofcarbonaceous matter obviously decreases with the temperature and oxygencontent. This gives a degree of freedom in the implementations of thepresent invention.

The present invention makes it possible to envisage the use of systemswhich are less constraining than the particle filter. It is possibleespecially to envisage systems having the characteristic of cyclones orany system which makes it possible to increase the duration of theresidence time in an atmosphere where the conditions necessary for theimplementation of this catalytic system prevail.

Under these conditions, if it is desired to remove the soot emitted bythe combustion systems, the said defined period of time is at most equalto the residence time of the particles in the space where the conditionsaccording to the present invention prevail constantly or intermittently.

It is thus possible to envisage a number of implementations of thepresent invention.

According to one of the implementations of the present invention, it ispossible to use a filter which collects the soot produced by thecombustion engine, while, if the need arises, boosts of air, enrichingthe exhaust gases with oxygen, make it possible, from time to time, atleast partially to regenerate the filter.

This enriching of the exhaust gases with oxygen, in order to reach avalue where the process according to the present invention isimplemented, is not necessary for all the operating modes of the engineand can be obtained by any means.

It can especially be enriched by the injection of air into the exhaustgas flow. This air can advantageously be heated by bringing into contactwith various heat sources and especially with hot components of theengine, directly or via a heat exchanger. The use can also be envisagedof heat reserves which will be heated during the period ofnon-regeneration of the filter by the exhaust gases and which wouldrestore the stored heat to the air which is charged with enriching theexhaust gases with oxygen during the periods of use of the processaccording to the present invention.

In certain cases, simply mixing the external air with the exhaust gasesmakes it possible to obtain a temperature and an oxygen content which issufficient to be placed in the region where catalytic oxidation takesplace under good conditions.

The conditions which prevail in the turbo of a diesel engine aresufficient for combustion to be obtained of the carbonaceous particleswhich, in general, contaminate the turbo of turbo engines. Indeed, in anentirely unforeseen way, it was shown that the addition of cerium, or ofany other rare-earth metal, made it possible to avoid any carbonaceousdeposition in the components of the turbo, thus facilitating theiroperation and their running, even though the temperature conditionswhich prevail in the turbo are very much lower than those which make itpossible to ignite soot, even soot doped with rare-earth metal.

The preferred rare-earth metals are cerium, neodymium and lanthanum.

They can be used alone or as a mixture with potentiating elements,including with other rare-earth metals. Reference may be made, mutatismutandis, to the part of the description directed more particularly atlanthanum.

However, to date, the best results are those obtained by means of ceriumand lanthanum, alone or in combination with other rare-earth metals.

In their catalytic function, these metals are preferably in the form oftheir IV oxide, in the case of cerium (and, for the sake of regularity,for praseodymium, but this is rather uneconomic), or of trivalent oxidein the other cases.

It is possible to use mixtures of rare-earth metals but, in this case,it is preferable for cerium and/or lanthanum to be in the majority.

It is also possible to use mixtures of rare-earth metals of the abovetype doped with non-rare-earth metal elements (cf. part on the doping oflanthanum, which part is transposable for any rare-earth metal ormixture of rare-earth metals).

The rare-earth metals can be introduced into the soot by introductioninto the motor fuel of a derivative such as their salts or of theirsols.

The introduction of compounds based on rare-earth metal(s) can becarried out especially by introduction of compounds based on rare-earthmetal(s) into the motor fuel intended to be introduced into the engine.

Another possibility consists in introducing the rare-earth metal(s) invarious forms via air, and especially air when it is mixed with exhaustgases of the engine, when a fraction of the exhaust gases is recycledinto the engine. In this case, the rare-earth metal(s) compoundsincorporated in the recycled soot are introduced into the engine.

However, it should be emphasized that the form under which therare-earth metal(s) is/are introduced into the fuel is not withouteffect on the size of the crystallites and aggregates which are found inthe soot, a size which plays a role in the catalytic ability of therare-earth metals present in the soot (cf. French Patent Application No.92/14158, filed on 25.11.1992 and entitled "Ceric oxide crystalliteaggregate, process of production and its use in reducing combustionresidues").

Moreover, the lifetime of cerium salts and sols in organic medium isoften low and can be the cause of poor results in the catalytic effect.Also, when it is desired to use cerium, it is preferable to use eitherthe sols or the salts which are the subject of European PatentApplication filed under No. 93/304760.7 and published under No.0,575,189. The addition to the motor fuel of the non-stable compounds atthe time of use makes it possible at least partly to overcome thedifficulties related to the non-stability (case of non-stabilizedcerium(III) salts).

Advantageously, the amount of rare-earth metals introduced into theengine is determined so that the rare-earth metal(s) content (by mass ofmetal contained) in the soot reaches a level of between 1000 ppm and30%, advantageously at least 5000 ppm and preferably at least 5% andadvantageously at most 25% and preferably at most 10%.

On average, during a European cycle, a satisfactory value is of theorder of 5 to 10%.

However, it should be emphasized that the form under which therare-earth metal(s) is/are introduced into the fuel is not withouteffect on the size of the crystallites and aggregates which are found inthe soot, a size which plays a role in the catalytic ability of therare-earth metals present in the soot (cf. French Patent Application No.92/14158, filed on 25.11.1992 and entitled "Ceric oxide crystalliteaggregate, process of production and its use in reducing combustionresidues"). In contrast to what could have been initially believed, thisteaching is valid not for cerium alone but for all the rare-earthmetals, alone or as a mixture, doped or undoped, and not as solelyrestricted to cerium. Thus, in the passage which follows and whichrelates to aggregates and to cerium (ceric) oxide crystallites of whichthey are composed, cerium oxide plays the role of paradigm (in the sameway that the verb "aimer"is the paradigm of the first French conjugationor that "dominus"is the paradigm of the second Latin declination) ofrare-earth metal oxides.

Thus, it is preferable to see to it that ceric oxide crystalliteaggregates are used directly or indirectly, in which aggregates thelargest size is between 20 Å (2 nanometers) and 10,000 Å(1000nanometers) and preferably between 50 Å (5 nanometers) and 5000 Å (500nanometers) and where the size of the crystallites, measured for the(1,1,1) plane by the Debye and Scherrer technique, is between 20 and 250Å (2 to 25 nanometers) and preferably from 100 to 200 nanometers.

It should be emphasized that these measurements are virtual measurementsand that it would doubtless be more correct to refer to the width of theX-ray peak.

For this reason, the technique for measuring the crystallite sizeaccording to the Scherrer technique will be indicated subsequently.

It should also be emphasized that the positional zeros are not, exceptwhen expressly indicated, significant figures.

It is preferable for these aggregates to be topologically as close aspossible to the soot, which is why it is desirable to introduce theaggregates into the combustion chamber or to manufacture in situ so thatthese aggregates can be formed simultaneously to serve as seeds for thesoot. In order to be effective, it is advisable to add, or to form "insitu", cerium oxide in the form of aggregates specified above at acontent of at least 10 ppm with respect to the carbonaceous fuel,preferably 20 ppm and more advantageously 50 ppm.

It is preferable that the cerium oxide thus formed has a particle sizesuch that the d₈₀ (diameter of the mesh which allows 80% by mass of theproduct to pass) is at most equal to 10,000 Å (1000 nanometers) andpreferably to 5000 Å (500 nanometers).

It is also preferable that the d₂₀ is greater than 200 Å (20 nanometers)and preferably 500 Å (50 nanometers).

The characteristics of the soot containing above aggregates can play arole in the present invention. It is therefore preferable that theparticle size of the soot is such that the grain has a d₂₀ equal to atleast 100 Å and/or a d₈₀ at most equal to 1000 Å and which contains atleast 0.01%, advantageously at least 0.1% and preferably at least 0.5%of aggregate according to the present invention.

Advantageously, on average, the rare-earth metal(s) content (by mass ofmetal contained) in the soot reaches a level of between 1000 ppm and30%, advantageously at least 5000 ppm and preferably at least 5% andadvantageously at most 25% and preferably at most 10%.

In general, the soot grains form masses in which the d₅₀ is between 2000and 5000 Å.

According to implementation of the present invention, the soot thusformed has a total cerium content between 1 and 5% by weight andpreferably of 1.5 to 2.5%.

As was mentioned above, the crystallite aggregate is advantageouslyformed during the combustion of the motor fuel, or of the fuel, thelatter being treated with at least one, preferably tetravalent, ceriumcompound in the solution or sol form.

It is also preferable that the d₂₀ is greater than 200 Å (20 nanometers)and preferably 500 Å (50 nanometers).

The factors which promote this low temperature combustion (orregeneration) are the oxygen content and the hydrocarbon compoundcontent.

As regards the oxygen content (which has already been dealt with above),it is preferable that the oxygen content is at least equal to 3% andpreferably to 5% approximately.

As regards the presence of hydrocarbon compounds in the exhaust gases,it is preferable that the volatility of the hydrocarbon compounds, theircontent in the gases and their temperature are such that, at atemperature where the soot is liable to be subjected to a combustion(for example is accumulated in the particle filter), the content (ratioby mass) of volatile hydrocarbon compounds in the soot is at least equalto one tenth, preferably a quarter and more advantageously a half of themass, on a dry basis.

Volatiles is understood to mean all the hydrocarbon compounds,especially those which exist in the exhaust gases, which are in the gasform at 600° C., advantageously at 400° C. and preferably at 350° C.

It is desirable that these hydrocarbon compounds have a boiling point ofbetween approximately 100° and 400° C.

This regeneration generally takes place when the content of hydrocarboncompounds in the gases is greater than or equal to 10 ppm and preferablygreater than 20 ppm.

The best results are obtained with diesel oils in which 95% by mass ofthe constituents distil under atmospheric pressure at a temperature atleast equal to 160° C. and advantageously to 180° C. and at which 95% bymass of the constituents are volatile under atmospheric pressure at 400°C. and preferably at 360° C.

The process gives better results with diesel oils containing a higharomatic content than with diesel oils containing a high aliphaticcontent, provided that the distillation constraints stated above areobserved.

Most often, the compositions are rare-earth metal(s) compounds which areliquid under the conditions of use (especially ambiant temperature atthe engine), in the form of sol(s) or dissolved, in hydrocarbondiluents, especially fuels, including diesel oils.

Thus, the present invention is particularly advantageous for two kindsof motor fuel. In the first, the aromatic content is very high contentof aromatic derivative(s) is at least equal to 1/3 and advantageously atleast equal to 1/3!, because it makes it possible to use these motorfuels which, without this invention, would lead to deposition whichwould be excessively problematic.

Moreover, it is on the motor fuels (that is to say the said mixtures)known as paraffinic, in which the paraffin content is at least equal to30%, that the effects are the most marked. These motor fuels are studiedfor the purpose of responding to new, more constraining standards. It isdesirable that, for this type of composition, the aromatic content (bymass) is at most equal to 1/5, advantageously to 1/10 and preferably to1/20.

The invention is particularly well suited to the particles emitted byfast diesel engines (in contrast to the slow engines). These engines arebasically used in ground transportation, such as heavy weights (lorries,coaches and the like), and light vehicles. Fast diesel engine isunderstood to mean engines in which the maximum power is reached atrotational speeds at least equal to 1500 revolutions/minute andadvantageously at least equal to 1800 revolutions/minute.

A particularly advantageous implementation of the present inventionconsists of a process for filtering the gases of an internal combustionengine, which process consists in:

introducing into the combustion chamber at least one derivative ofrare-earth metals or of a mixture of rare-earth metals at aconcentration of between 10 ppm and 500 ppm (by mass), preferablybetween 20 ppm and 200 ppm;

collecting the soot produced by the internal combustion engine on afilter, the temperature of the gases entering into the filter beingchosen within the range 100° C.-350° C. (the positional zeros are notsignificant figures); and

leaving the soot to accumulate until a speed is reached where asignificant fraction of the soot arriving is compensated for by thecombustion of soot in the soot cake on the filter and providing for noregeneration as long as the pressure drop caused by the soot does notexceed a value chosen in advance and not exceeding 400 millibar.

The above pressure drop does not incorporate the pressure dropoccasioned by the unladen soot filter, which pressure drop is in generalless than 100 millibar and very often less than 50 millibar.

For reasons which are not entirely clear, metal filters giveparticularly good results or more precisely particularly frequentregenerations.

The filters give better results after 3 and preferably 5 regenerationcycles.

It could surprisingly be observed that there was, in general, no need tocause regenerations when the above conditions were respected.

Allogenous (or exogenous) regeneration is not as a general rule usefuland must only be envisaged when it is desired to maintain a particularlylow level of pressure drop (generally less than 200 or indeed 150millibar).

It is then advisable, as allogenous regeneration, to envisageregenerations carried out by non-autogenous localized overheating (inorder to bring the point to a temperature at least equal to 500° C. andadvantageously to 600° C.) and not by overall overheating over the wholeof the filter; this possibility is only advantageous because, accordingto the invention, localized overheating does not lead to overall andcomplete regeneration of the filter.

Localized overheatings can be carried out by any means known to thoseskilled in the art, such as microresistor(s) distributed over thesurface of the filter, metal particles heated by eddy currents, miniarcor equivalent means.

The filtration mode according to the invention generally only operatesduring some of the speeds of an engine, because some of the speedsgenerate gases with a temperature at least equal to approximately 500°C. (two significant figures) which, ipso facto, produces progressive andoften complete regeneration of the filter.

It should be noted that the temperature of the gases entering into thefilter can vary widely within the temperature range 100°-400° C. Thesevariations, which can be deliberately caused, promote the regenerationand make it possible to maintain a pressure drop which is low in value.

The position of the filter can advantageously be chosen so that thetemperature of the filter is for the greatest possible portion of thetime at a temperature within the above ranges.

As indicated above the preferred rare-earth metals are cerium, lanthanumand the mixtures containing cerium and lanthanum. The most commonrare-earth metal content (metal content) of the motor fuel is between 50and 150 ppm.

According to the present invention, it is possible to choose therare-earth metals content of the motor fuel so as to adjust the pressuredrop to a value chosen in advance. It is also possible to act on thefiltering surface, while remaining within the above-specifiedtemperature range.

This value chosen in advance is preferably between 100 and 400 millibar,preferably between 150 and 300 millibar.

In order to obtain such results, it is preferable that the rare-earthmetals present in the soot have a concentration between 500 ppm and 10%,preferably between not less than 1000 ppm (mass of metal content withrespect to the total mass of the soot, including the compounds which ithas adsorbed) and not more than 5%, on average.

In the majority of currently marketed engines, it is advantageous tointroduce the rare-earth metal, or the mixture of rare-earth metals,with its retinue of impurity(ies) and adjuvant(s), at a content ofbetween 10 and 1000 ppm into the motor fuel. These values are expressedas metal content. Advantageously, contents of 20 to 200 ppm, preferablyof 50 to 150 ppm, are used.

Some internal explosion engines, such as petrol engines and some dieselengines in the course of testing and development, produce or shouldproduce less soot. Insofar as only the effect of regenerating the filteris being sought (and not a general improvement in the combustion), thismakes it possible to reduce the amounts of rare-earth metal, or ofmixture of rare-earth metals, to be introduced into the motor fuel, whenthis method of introduction has been favoured, and more generally intothe combustion chamber. This reduction is carried out pro rata withrespect to soot production in order to maintain the content ofrare-earth metals, or the mixture of rare-earth metals, in the saidsoot.

It is advantageous that the form under which the rare-earth metal, orthe mixture of rare-earth metals, is introduced leads to the formationof aggregates of crystallites of the rare-earth metal oxide, or ofoxides of the mixture of rare-earth metals, where the largest size ofthe said aggregate is between 20 angstroms and 10,000 angstroms,preferably between 100 and 5000 angstroms, and for which the size of thecrystallites is between 20 and 250 angstroms, preferably between 50 and200 angstroms.

Introduction of the compounds based on rare-earth metals, alone or as amixture, can be carried out especially by introduction of compoundsbased on rare-earth metals, alone or as a mixture, into the motor fuelintended to be introduced into the engine.

Another possibility consists in introducing the rare-earth metals, aloneor as a mixture, in various forms via air, and especially air when it ismixed with exhaust gases of the engine, when a fraction of the exhaustgases is recycled in the engine. In this case, the compounds of therare-earth metals, alone or in a mixture, incorporated in the recycledsoot are introduced into the engine.

One of the most widely used means for introducing the rare-earth metal,alone or as a mixture, into the engine circuit consists in introducingit into the motor fuel either in the salt form or in the sol form. Thesesalt or sol compounds advantageously contain products which are harmlessfor combustion and for the environment.

Thus, it is preferable that the salts or the sols are prepared fromhydrocarbon compounds such as the salts of carbonaceous acids, whetherthey are of the carboxylic acid type or of the mobile hydrogen compoundtype such as, for example, acetylacetonates.

It is also possible to envisage compounds of acid type based on sulphursuch as sulphuric acids (alkyl or aryl acid sulphates) or else acids ofsulphonic type. However, the latter acids have the disadvantage ofincreasing the sulphur content of the motor fuel.

Generally, for rare-earth metals having a valency of III as the highestvalency, the salts of C₂ to C₂₀, preferably from C₄ to C₁₅, carboxylicacid are among the best suited to this use.

For cerium, the derivatives of cerium(IV) are preferred due to theirstability and their ability to produce few particles.

It is preferable that the rare-earth metal oxides or mixtures ofrare-earth metal oxides are stable in the motor fuel. Cerium is thepreferred rare-earth metal, alone or in combination. Cerium can beintroduced into the motor fuels either in the form of sols or in theform of various salts, provided that the latter are sufficiently stablein the medium. Mention may especially be made of the salts which are thesubject of the European patent application filed under the number93/304760.7 and published under the number 057 5189.

In fact, in an entirely surprising way, it could be observed that, whenadditives based on transition metals (that is to say metals in which oneof the d subshells is in the course of filling) were used,low-temperature regenerations were either nonexistent or else gave riseto violent ignitions leading to high temperatures capable of damagingthe filters.

In contrast, in the case of components to which derivatives based onrare-earth metals have been added, a state is fairly rapidly reached,after an accumulation phase in the filter, in which the amount of sootarriving on the filter is compensated for by many random, but notviolent, combustions which have taken place in the body of the sootaccumulated on the filter.

The result is thermal effects which are not very pronounced as well aspressure drop variations which are much smaller.

Thus, it was possible to show that the use of additives based onrare-earth metal elements leads to a two-fold advantage:

in the first place, the limitation in the thermal deviations (due to theregeneration of the filter) makes it possible for the filtering materialto retain all its properties and especially its filtration efficiency;the system has a longer lasting and better efficiency;

in the second place, the behavior in the presence of additives based onrare-earth metal elements makes it possible for the pressure dropphenomena to be more flexibly managed; in fact, in the case of additivesbased on transition-metal elements (especially on copper and iron), theregenerations are random, fierce and violent, the pressure dropschanging very abruptly. This leads to variations in the power of theengine and harms the safety and comfort of driving as well as thesatisfactory operation of the engine; on the other hand, in the case ofadditives based on rare-earth metal elements, the regenerations are lowin magnitude, which reduces the effects on the pressure drop and thethermal effects. The pressure drop due to the filter stabilizes and canbe managed without penalizing the safety and agreeableness of driving.

According to one of the preferred embodiments of the present invention,it is possible to use lanthanum derivatives for implementing the presentinvention. This has led to the development of lanthanum derivatives aswell as of other doped rare-earth metals.

The invention provides a process for reducing the emission of soot froman internal combustion engine, in which the exhaust gases are made topass through a particle filter and in which an additive containinglanthanum is introduced into the combustion chambers.

According to one of the embodiments of the present invention, lanthanumis added to the motor fuel in the form of a salt or of a stable sol.

The use of rare-earth metals as combustion adjuvants has been describedfor a long time in the state of the prior art but lanthanum has onlyever been mentioned incidentally, or rather accidentally, as a member ofthis family.

More recently, in an exhaustive study carried out at the InstitutFrangais du Petrole on various rare-earth metals and on their ability tocatalyse the oxidation of soot, M. Desoete has shown that lanthanum hadno catalysis property on carbonaceous particles.

This article, published in English and entitled "Catalysis of sootcombustion by metal oxides", dated February 1988 and published underreference 35991 (available on public request), clearly shows, in FIG.10, the absence of an effect by lanthanum oxide.

In an entirely surprising way, the soot formed by the introduction oflanthanum-based compounds into the circuits of the internal combustionengines, and especially diesel engines, has the property of beingsignificantly easier to ignite, that is to say having a lower ignitiontemperature, than the soot prepared without additives.

Introduction of the lanthanum-based compounds can be carried outespecially by introduction of lanthanum-based compounds into the motorfuel intended to be introduced into the engine.

Another possibility consists in introducing lanthanum in various formsvia air, and especially air when it is mixed with exhaust gases of theengine, when a fraction of the exhaust gases is recycled in the engine.In this case, the lanthanum compounds incorporated in the recycled sootare introduced into the engine.

One of the most widely-used means for introducing lanthanum into theengine circuit consists in introducing it into the motor fuel either inthe salt form or in the sol form. These salt or sol compoundsadvantageously contain products which are harmless for combustion or forthe environment.

Thus, it is preferable that the salts or the sols are prepared fromhydrocarbon compounds such as the salts of carbonaceous acids, whetherthey are of the carboxylic acid type or of the mobile hydrogen compoundtype such as, for example, acetylacetonates.

It is also possible to envisage compounds of acid type based on sulphursuch as sulphuric acids (alkyl or aryl acid sulphates) or else acids ofsulphonic type. However, the latter acids have the disadvantage ofincreasing the sulphur content of the motor fuel.

Generally, the salts of C₂ to C₂₀, preferably of C₄ to C₁₅, carboxylicacid are among the best suited to this use.

In order to obtain good catalysis of the combustion of the soot, it isadvisable to provide for a satisfactory concentration of lanthanum andof its optional potentiation adjuvants in the soot; this concentrationis advantageously between 500 ppm and 10%, preferably between at least1000 ppm (mass of metal content with respect to the total mass of thesoot, including the compounds which it has adsorbed) and at most 5%, onaverage.

In the majority of currently marketed engines, it is advantageous tointroduce lanthanum, with its retinue of impurity(ies) and adjuvant(s),at a content of between 10 and 1000 ppm into the motor fuel. Thesevalues are expressed as metal content. Advantageously, contents of 20 to200 ppm, preferably of 50 to 150 ppm, are used.

Some internal explosion engines, such as petrol engines and some dieselengines in the course of testing and development, produce or shouldproduce less soot. Insofar as only the effect of regenerating the filteris being sought (and not a general improvement in combustion), thismakes it possible to reduce the amounts of lanthanum to be introducedinto the motor fuel, when this method of introduction has been favoured,and more generally into the combustion chamber. This reduction iscarried out pro rata with respect to soot production in order tomaintain the lanthanum content in the said soot.

It is advantageous that the form under which lanthanum is introducedleads to the formation of aggregates of crystallites of lanthanum oxidewhere the largest size of the said aggregate is between 20 angstroms and10,000 angstroms (2 and 1000 nm), preferably between 100 and 5000angstroms (10 and 500 nm), and for which the size of the crystallites isbetween 20 and 250 angstroms (2 and 25 nm) and preferably between 50 and200 angstroms (5 and 20 nm).

According to the present invention, it has also been shown thatlanthanum was an element capable of potentiating, or of beingpotentiated by, other elements especially capable of catalysingoxidations of carbonaceous products and those leading to defects in thecrystalline lattice of lanthanum oxide.

It has thus been shown that transition elements, that is to say metalsin which one of the d subshells is in the course of filling, gave markedsynergic effects with lanthanum. This synergic effect is alsodemonstrated with other elements containing f shells in the course offilling, and especially with the other rare-earth metals, includingyttrium.

The most marked results are those of lanthanum in combination withmanganese, copper, cobalt and/or iron. Other rare-earth metals,including yttrium, alone or as a mixture, also have a specificadvantage.

The lanthanum content, with respect to the sum of the metal elementscontained in the adjuvant, is generally between 5% and 95%.Advantageously, it is at least equal to 50% and preferably to 80%.

The elements which potentiate, or are potentiated by, lanthanum areintroduced in the same way as this element can be introduced.

As was mentioned above, another aim of the present invention is toprovide a process which makes possible a regeneration of the particlefilters which can be described as low temperature.

This aim is achieved by means of a process using the lanthanum-basedcompounds mentioned above.

This process consists in:

introducing a lanthanum derivative as specified above into the motorfuel at a concentration of between 10 ppm and 500 ppm (by mass) andpreferably between 20 ppm and 200 ppm (as metal content);

collecting the soot produced by the internal combustion engine on afilter, the temperature of the gases entering into the filter beingchosen within the range 100°-400°C. (in the present description, thepositional zeros are not significant figures, except when this isspecified), and leaving the soot to accumulate until a speed is reachedwhere a significant fraction of the soot arriving is compensated for bythe combustion of soot in the soot cake on the filter and providing forno imposed regeneration as long as the pressure drop caused by the sootdoes not exceed a value chosen in advance and advantageously notexceeding 500 millibars.

Indeed, according to the present invention, it was possible to show thatit was possible to have regenerations at temperatures as low asapproximately 100° C. During a European engine cycle, it is possible tohave violent temperature variations in exhaust gases which can, withoutreaching the above maximum value (that is to say, the minimumtemperature which is fatal to the filter(s)), make it possible to obtainpartial or complete regeneration.

The following non-limiting examples illustrate the invention.

EXAMPLES

A-- Description of the experimental conditions

The engine used is a four-cylinder, indirect injection, atmosphericdiesel engine with a cubic capacity of 1.696 liters and developing 50kilowatts at 4400 revolutions per minute. This engine is sold under thetrade name Volkswagen.

The filters used are cordierite filters produced by the Company Corning,of the EX 4-7 type (5.66 inches in diameter, 6 inches long, with a celldensity of 100 cpi/17 mil). Each additive was tested on a fresh filter.The following are measured continuously during the tests:

the pressure drop related to the filter (pressure drop between the inletand outlet of the filter);

the temperature of the gases at the inlet of the filter;

the temperature of the gases at the outlet of the filter;

the carbon monoxide emissions.

The tests are carried out at 2000 revolutions/minute while maintainingthe temperature of the gas at the inlet of the filter constant withtime. The tests carried out at a temperature of 250 ° C. are reportedbelow but similar results were obtained at other temperatures.

The tests were carried out with especially:

an iron-based additive whose content in the fuel oil is 20 ppm (mass);

a copper-based additive; the copper level in the fuel oil is 20 ppm;

two additives based on cerium and rare-earth metals; the content ofrare-earth metal elements (cerium) in the fuel oil is 50 ppm.

Taking into account the respective molar masses of the elements, themolar, or more exactly atomic, contents are substantially of the sameorder for all the additives, namely:

iron: 0.36 mol/1000 kg of fuel oil;

copper: 0.32 mol/1000 kg of fuel oil;

cerium: 0.36 mol/1000 kg of fuel oil.

B-- Results

Example No. 1

the case of the iron-based additive

The results are reported in FIG. 1. This figure gives, on the one hand,a change in the pressure drop as a function of time and, on the otherhand, the carbon monoxide content in the gas at the filter outlet aswell as the change in the temperature of the gas at the filter outlet asa function of time. It is observed that the periods of accumulation canreach a duration of 35,000 seconds. The pressure drop easily reaches 300millibar. During the regenerations which correspond to the violentreduction in the back pressure, a strong increase in the carbon monoxidecontent of the gases at the filter outlet is observed. Simultaneouslywith these variations in carbon monoxide content, a very abrupt increasein the temperature of the gases at the filter outlet is observed, withtemperatures which can reach 700° C. or indeed 800° C., whereas thetemperature of the gases at the inlet of the filter is only 250° C. Thevariation in the back pressure is very violent. It is possible to lose250 millibar very quickly during these regeneration phases and theduration of the accumulation regions, as with the magnitude of the backpressure variations, seems to vary randomly. These graphs demonstratethe random and violent behavior of the regenerations, which allows it tobe supposed that the engine will be doubtless affected by these abruptdownstream back pressure variations.

In this example, the iron-based additive is ferrocene.

Example No. 2

the case of the copper-based additive

The copper used is a cupric carboxylate. The results are reported inFIG. 2. This figure gives, on the one hand, the change in the backpressure as a function of time and, on the other hand, the carbonmonoxide content in the gas at the filter outlet and the change in thetemperature of the gas at the filter outlet, as a function of time.

It is observed that the periods of accumulation can reach 54,000seconds, i.e. nearly 20 hours. A pressure drop, or back pressure, whichcan reach 350 millibar is observed. During the regenerations whichcorrespond to the abrupt reduction in the pressure drop, a strongincrease in the carbon monoxide content in the gases at the filteroutlet is observed. In parallel with these variations in the carbonmonoxide content, a very abrupt increase in the temperature of the gasesat the filter outlet is observed, with temperatures which can reach morethan 800° C., whereas the temperature of the gases at the filter inletis only 250° C. The variation in the back pressure is very violent. Itis possible to lose 300 millibar very quickly during these regenerationphases. The duration of the accumulation regions, as with the magnitudeof the variations in back pressure, seems to vary randomly. These graphsdemonstrate the random and violent behavior of these regenerations,which allows it to be supposed that the engine will be affected by theseabrupt downstream back pressure variations.

Example No. 3

the case of a cerium-based additive (compound described in the Europeanpatent application filed under the number 93/304760.7 and publishedunder the number 057 5189)

The results are reported in FIG. 3. This figure gives, on the one hand,the change in the back pressure as a function of time and, on the otherhand, the CO content in the gas at the filter outlet and the change inthe temperature of the gas at the filter outlet as a function of time. Aperiod of charging the filter of 25,000 seconds is observed. Beyond thispoint it is observed that the back pressure is virtually stable. Thisback pressure stabilizes in the region of 200 millibar. At the time ofthe regenerations which correspond to very low decreases in the backpressure (less than 50 millibar), variations in the carbon monoxidecontent in the gases at the filter outlet are observed. These variationstestify to a constant regeneration activity with time. Parallel to thesevariations in the carbon monoxide content, a moderate increase in thetemperature of the gases at the filter outlet is observed, withtemperatures which can reach not more than 400° C., for a temperature ofthe gases at the filter inlet of 250° C. These graphs demonstrate thebehavior, which is both moderate and permanent, of the regenerations,which allows it to be supposed that the engine and thus theagreeableness of driving is not thereby detrimentally affected.

Example No. 4

example of an additive based on a cerium sol

These results are reported in FIG. 4. This figure gives, on the onehand, a change in the back pressure as a function of time and, on theother hand, the carbon monoxide content in the gas at the filter outletand the change in the temperature of the gas at the filter outlet as afunction of time. A period of charging the filter of 45,000 seconds isobserved. Beyond this time, it is observed that the back pressure isvirtually stable. This back pressure stabilizes in the region of 200millibar. At the time of the regenerations which correspond to verysmall decreases in the back pressure (less than 50 millibar), variationsin the carbon monoxide content in the gases at the filter outlet areobserved. These variations testify to a constant regeneration activitywith time. In parallel with these variations in the carbon monoxidecontent, a moderate increase in the temperature of the gases at thefilter outlet is observed, with temperatures which can reach not morethan 350° C., for a temperature of the gases at the filter inlet of 250°C. These graphs demonstrate the behavior, which is both moderate andpermanent, of the regenerations, which allows it to be thought that theengine, the safety and the agreeableness of driving are not therebydetrimentally affected in any way. The number of fine particles formedis significantly less than that in the preceding examples.

Example No. 5

tests on a constant-speed engine bed

Tests were carried out on a new engine of F8Q 706 type with a capacityof 1870 cm³ and the particle filter used is an Eberspacher particlefilter number 2626000415. This filter is equipped with twothermocouples, one upstream and the other downstream. The tests werecarried out on particle filters which have been subjected to a number ofchargings and a number of regenerations. In fact, the first effects arerelatively erratic.

For each additive: a fresh particle filter (PF) is used.

The particle filter must be initially stabilized, that is to say that itmust be subjected to a minimum of 3, fouling and regeneration, cycles.

Fouling at the 1500 rev/min 3/4 load position, charging of the particlefilter with 70 g of soot;

Regeneration at the 4000 rev/mim position, exhaust temperature: 600° C.

The following law is found for each particle filter, during thestabilization phase in the process of fouling, and for the foulingposition: mass of soot deposited: f(ΔP PF) (the relationship isconstructed by successively removing and weighing the filter element).

The self-regeneration test is carried out as follows:

Starting conditions:

The particle filter is fouled with 70 g of soot,

Before the test, the particle filter is cold (ambient temperature).

The tests below are tests on stabilized particle filters (PF).

The tests are carried out at constant speed, the speed being expressedby number of revolutions to the minute.

The results obtained, either without additive or with an additive basedon various rare-earth metal salts introduced into the motor fuel in aproportion, except if otherwise indicated, of 100 ppm of metal content,are collated in the following tables.

The filters are charged beforehand with particles under conditions wherethey accumulate even in the presence of rare-earth metals. Theseconditions are:

1500 revolutions/minute;

3/4 of the full load;

richness of the mixture: between 0.8 and 0.9

oxygen content of the gases: between 2 and 4%;

the particle filters are charged with 70 g of soot per filter.

Characterization of the self-regeneration ability of soot with anadditive-containing diesel oil on an F80 706 engine

The test lasts approximately 130 minutes (8000 seconds). It could beobserved that, when regeneration took place, the total pressure wasmaintained at a value of less than approximately 200 millibar during theremainder of the test. The pressure drop related to the filter alone isof the order of 50 to 100 millibar.

Constant speed

The self-regeneration tests are carried out at three constant speeds inthe order: 2500, 1500 and 4000 rev/min.

The engine is switched on and adjusted to the operating position(Example: 2500 rev/min, MEP (Mean Effective Pressure) 0 bar, whilewaiting for the engine to warm up, after 10 minutes, beginning theacquisition of the first position. The test takes place by increasingthe load every ten minutes according to predefined stationary stages(MEP: 0, 1, 2, 3, 4, 4.5, 5, 5.5, 6.5, 7 and 7.5 bar).

The mean effective pressure (MEP) is given from the torque by therelationship MEP= (40×π×torque)/cubic capacity.

Once the particle filter is charged, the engine is set at a certainspeed and, as specified above, the load is progressively increased bystationary stages of 10 minutes (increment of the order of 9 N.m perstationary stage) until the value of the order of 100 N.m is reached.Once regeneration is obtained, the values of the different variablesjust before triggering are determined.

No-load test

The engine is run under no load at the desired speed.

                                      TABLE 1                                     __________________________________________________________________________    Tests without additive                                                                T.                                                                            upstream dp                with or                                    SPEED                                                                             Exh. T.                                                                           PF   MEP PF  O.sub.2                                                                          HC    CO   without                                    rev/min                                                                           °C.                                                                        °C.                                                                         bar mbar                                                                              %  ppm                                                                              g/h                                                                              ppm                                                                              g/h                                                                             additive                                   __________________________________________________________________________    1500    503  4.15                                                                              769 -- -- -- -- --                                                                              without                                    2500                                                                              432 382  4.15                                                                              853  9.2          without                                    4000                                                                              451 419  1.55                                                                              1154                                                                              11.2          without                                    __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________    NO-LOAD TEST                                                                  with 150 ppm (metal content) of cerium salt (cerous octoate)                          T.                                                                        Exh.                                                                              upstream                                                                              dp                 with or                                    SPEED                                                                             T.  PF   MEP                                                                              PF  O.sub.2                                                                          HC    CO    without                                    rev/min                                                                           °C.                                                                        °C.                                                                         bar                                                                              mbar                                                                              %  ppm                                                                              g/h                                                                              ppm                                                                              g/h                                                                              additive                                   __________________________________________________________________________     800                                                                               98 79   0  240 17.50                                                                            138                                                                              2.8                                                                              300                                                                              12.3                                                                             with                                       1000                                                                              104  87* 0  258 17.45                                                                            62 1.5                                                                              230                                                                              11.5                                                                             with                                       1500                                                                              126 114* 0  360 17.70                                                                            38 1.60                                                                             250                                                                              21.7                                                                             with                                                                       0                                             1500                                                                              125 119* 0  426 17.55                                                                            27 1.1                                                                              240                                                                              1.2                                                                              with                                       2500                                                                              239 227* 0  1156                                                                              15.55                                                                            58 3.8                                                                              320                                                                              42.6                                                                             with                                       4000                                                                              362 336* 0  1133                                                                              13.45                                                                            46 4.6                                                                              220                                                                              44.7                                                                             with                                       __________________________________________________________________________

Results on lanthanum

The results obtained with an additive based on lanthanum octoateintroduced into the motor fuel in portion of 100 ppm of lanthanum arecollated in the following tables:

                                      TABLE 3                                     __________________________________________________________________________    TESTS CARRIED OUT AT CONSTANT SPEED WITH LANTHANUM SALT                               T.                                                                            upstream                                                                              dp                 with or                                    SPEED                                                                             Exh. T.                                                                           PF   MEP                                                                              PF  O.sub.2                                                                          HC    CO    without                                    rev/min                                                                           °C.                                                                        °C.                                                                         bar                                                                              mbar                                                                              %  ppm                                                                              g/h                                                                              ppm                                                                              g/h                                                                              additive                                   __________________________________________________________________________    1500                                                                              460 388  6.4                                                                              705 5  10 0.4                                                                              150                                                                              12 with                                       no regeneration                                                               1500 d                                                                            295 241* 2.5                                                                              663 13.5                                                                             46 1.9                                                                              160                                                                              13.4                                                                             with                                       a 2500                                                                            371 331* 3.5                                                                              821 10.8                                                                             29 1.9                                                                              140                                                                              18.3                                                                             with                                       b 2500                                                                            285 251* 2.2                                                                              943 13.8                                                                             32 2.2                                                                              170                                                                              23.1                                                                             with                                       4000                                                                              431 393* 1.6                                                                              1060                                                                              10.9                                                                              8 0.8                                                                              220                                                                              46.8                                                                             with                                       __________________________________________________________________________

                                      TABLE 4                                     __________________________________________________________________________    NO-LOAD TESTS WITH LANTHANUM SALT                                                     T.                                                                            upstream                                                                              dp                 with or                                    SPEED                                                                             Exh. T.                                                                           PF   MEP                                                                              PF  O.sub.2                                                                          HC    CO    without                                    rev/min                                                                           °C.                                                                        °C.                                                                         bar                                                                              mbar                                                                              %  ppm                                                                              g/h                                                                              ppm                                                                              g/h                                                                              additive                                   __________________________________________________________________________    1500                                                                              498 124  0  705 17.5                                                                             46 2.00                                                                             265                                                                              23.00                                                                            with                                       1500                                                                              189 161  0  892 16.8                                                                             410                                                                              16.8                                                                             660                                                                              54.8                                                                             with                                       Al = 0                                                                        no regeneration                                                               1500                                                                              170  145*                                                                              0  1020                                                                              17 82 3.5                                                                              240                                                                              20.5                                                                             with                                       __________________________________________________________________________     *spontaneous regeneration                                                

TESTS ON ARTIFICIAL SOOT

Method for producing the soot

The soot is obtained by pyrolysis of fuel oil containing the additive. Agas flow consisting of a 98/2 (by volume) mixture of nitrogen and oxygenis passed through a tube known as a "reactor tube". The reactor tube isheated by virtue of an oven and the gas flow is thus brought to 1200° C.The fuel oil, with or without additive, is sprayed very finely upstreamof the oven. The fuel oil droplets are transported by the gas flow andare also brought to 1220° C.

Under these conditions, the fuel oil partially burns and forms soot. Thesoot is then collected downstream of the pyrolysis oven by filtering thegas flow. If the fuel oil contains a metal additive, the metal is foundin the oxide form, intimately mixed with the soot. The flow rate ofinjection of the fuel oil upstream of the pyrolysis oven is adjusted to10 ml/h.

Under these conditions, the mean diameter of the elemental particles ofthe soot produced by pyrolysis is identical to that of the soot producedby a diesel engine.

According to studies carried out by the Applicant company, the metaladditives are used at a concentration in the engine diesel oil such thatthe concentration of metal in the diesel oil is between 0 and 200 ppm.Under these conditions, the concentrations of metal in the soot producedby an engine fed with an additive-containing diesel oil is between 0 and4%.

As the final concentration of catalyst in the soot is one of theimportant parameters which condition the ignition temperature, it wasnecessary to search for that concentration of metal in the fuel oilwhich it was advisable to have in order to obtain the same concentrationof metal in the soot obtained by pyrolysis.

It was shown that it is necessary for the concentration of metal in thefuel oil to be multiplied by 20 with respect to the concentration ofadditive in the engine diesel oil. The range is thus of the order of 0to 4000 ppm by mass.

in conclusion, for the soot obtained by pyrolysis to be representativeof the engine soot, the concentration of additive in the fuel oil mustbe multiplied by 20.

Studies of the soot by Thermogravimetric analysis or TGA

The soot produced by the pyrolysis oven can be recovered in TGA.

The conditions are the following:

    ______________________________________                                        Equipment        Setaram: TG92 model                                          Gas flow rate    2.3 1/h                                                      Gas              air                                                          Temperature      increasing from 20 to 900° C. at                                       the rate of 10° C./min                                Mass of soot analysed                                                                          20 mg                                                        ______________________________________                                    

TGA records the mass of soot remaining as a function of time. Byconvention, the ignition temperature is defined by us as the abscissa ofthe point defined as the intersection of the base line to the originwith the tangent to the curve at a point where the rate of combustion ismaximum (that is to say, at the point of inflection of the so-called "S"curve).

Study of the reactivity of the soot obtained by pyrolysis of diesel oilscontaining additives composed of lanthanum, copper and cobalt

Copper and cobalt are two metals which are known for their ability tolower the ignition temperature, are reputed to be toxic and inducethermal shocks which are dangerous for the filter. One of the subjectsof the present study was to show the synergy which exists between ametal such as copper or cobalt with respect to lanthanum.

The soot samples were all produced by the method described above andtheir tendency towards combustion was measured by TGA. Theconcentrations of metal in the fuel oil are certainly very high but theygive information on an implementation on an engine bed withconcentrations in the diesel oil used as motor fuel corresponding toapproximately 1/20th of the concentrations in the fuel oil.

Additives used

The names of the additives used for this study, and theircharacteristics, are combined in Table 1:

                  TABLE 5                                                         ______________________________________                                        Characteristic of the additives used                                                         Concentration of the metal in                                  Name of the additive                                                                         the additive as %                                              ______________________________________                                        Copper Cekanoate                                                                             9.2                                                            Cobalt octoate 10                                                             Lanthanum octoate                                                                            10.23                                                          ______________________________________                                    

The ignition temperature of the soot is given in the tables below as afunction of the concentration of metal in the fuel oil.

                  TABLE 6                                                         ______________________________________                                        Ignition temperature as a function of the concentration                       of metal acting alone in the fuel oil                                         (comparative examples)                                                        CONCENTRATION OF METALS TEMPERATURE                                           IN THE FUEL OIL (mmol/kg)                                                                             (in °C.)                                                        Adjuvant of                                                                              Total Of                                          TEST No.                                                                             Lanthanum the lanthanum                                                                            conc. ignition                                                                            Lowering                              ______________________________________                                        1                         0         490   0                                   2                Copper:  3.04                                                                              3.04  470   -20                                 3                Copper:  7.14                                                                              7.14  375   -115                                4                Copper: 10.7 10.7  370   -120                                5                Copper: 14.2 14.2  360   -130                                6                Copper: 17.8 17.8  360   -130                                7                Copper: 26.8 26.8  355   -135                                8                Copper: 28.5 28.5  355   -135                                9                Cobalt:  3.56                                                                              3.56  435   -55                                 10               Cobalt:  7.12                                                                              7.12  375   -115                                11               Cobalt: 10.7 10.7  355   -135                                12               Cobalt: 14.3 14.3  365   -125                                13               Cobalt: 17.8 17.8  335   -155                                ______________________________________                                    

                  TABLE 7                                                         ______________________________________                                        Ignition temperature as a function of the concentration                       of lanthanum acting alone in the fuel oil                                     CONCENTRATION OF METALS                                                       IN THE FUEL OIL (mmol/kg)                                                                             TEMPERATURE                                                             Total (in ° C.)                                                       Adjuvant of                                                                              concen-                                                                             Of                                          TEST No.                                                                             Lanthanum the lanthanum                                                                            tration                                                                             ignition                                                                            Lowering                              ______________________________________                                        14     14.3      0          14.3  410   -80                                   15     38.9      0          38.9  400   -90                                   ______________________________________                                    

                  TABLE 8                                                         ______________________________________                                        Ignition temperature as a function of the composition                         of the adjuvants in the fuel oil                                              CONCENTRATION                                                                 OF METALS IN THE                                                              FUEL OIL                TEMPERATURE                                           (mmol/kg)               (in °C.)                                                        Adjuvant of                                                                              Total Of                                          TEST No.                                                                             Lanthanum the lanthanum                                                                            conc. ignition                                                                            Lowering                              ______________________________________                                        16     7.14      Copper: 3.56                                                                             10.7  380   -110                                  17     7.14      Copper: 7.14                                                                             14.3  375   -115                                  18     7.14      Copper: 10.7                                                                             17.8  370   -120                                  19     10.7      Copper: 3.56                                                                             14.3  400    -90                                  20     8.53      Copper: 7.14                                                                             15.7  360   -130                                  21     14.3      Copper: 3.56                                                                             17.8  380   -110                                  22     9.03      Copper: 8.83                                                                             17.8  370   -120                                  23     5.40      Copper: 5.28                                                                             10.7  405    -85                                  24     9.00      Copper: 26.8                                                                             35.8  340   -150                                  25     13.44     Cobalt: 12.18                                                                            25.62 360   -130                                  26     5.62      Cobalt: 5.09                                                                             10.71 377   -113                                  27     4.70      Cobalt: 6.11                                                                             10.81 360   -130                                  28     7.14      Cobalt: 3.56                                                                             10.70 395    -95                                  29     7.14      Cobalt: 7.14                                                                             14.28 365   -125                                  30     7.14      Cobalt: 10.71                                                                            17.85 355   -135                                  31     10.75     Cobalt: 3.56                                                                             14.31 350   -140                                  32     10.75     Cobalt: 7.14                                                                             17.89 360   -130                                  33     14.28     Cobalt: 3.56                                                                             17.84 375   -115                                  ______________________________________                                    

We claim:
 1. Process for the treatment of soot produced by an internalcombustion engine, containing one or more rare-earth metals wherein saidprocess comprises:introducing into a combustion chamber of saidcombustion engine at least one derivative of a rare-earth metal or of amixture of rare-earth metals at a concentration of between 10 ppm and500 ppm, by mass; collecting soot produced by the internal combustionengine on a filter; contacting said soot with a gas containing oxygen,wherein said gas has a temperature of between 100° C. and 350° C. andthe oxygen partial pressure of said gas containing oxygen is at leastequal to 0.03 atmospheres; and allowing the soot to accumulate until aspeed is reached where a significant fraction of the soot arriving iscompensated for by the combustion of soot in the soot cake on the filterand providing for no regeneration as long as the pressure drop caused bythe soot does not exceed a predetermined value not exceeding 0.5 bar. 2.Process according to claim 1, characterized in that the contact betweenthe gas containing oxygen and the soot is maintained for a time intervalwhich is sufficient to burn at least 90% of the soot.
 3. Processaccording to claim 1, wherein at least a part of the said gas containingoxygen arises from air heated in contact with a part of the engine. 4.Process according to claim 1, wherein the gas containing oxygen at leastpartially contains the exhaust gases of the engine.
 5. Process accordingto claim 1, wherein said engine is a diesel engine.
 6. Process accordingto claim 1, wherein the engine is equipped with a turbocharger. 7.Process according to claim 1, wherein the rare-earth metals are presentin the soot at a level of between 500 ppm and 5%.
 8. Process accordingto claim l, wherein the particles contained in the filter areintermittently subjected to the conditions of the present process. 9.Process according to claim 3, wherein said rare-earth metal isintroduced into the engine via air.
 10. Process according to claim 3,wherein said rare-earth metal is introduced into the engine via themotor fuel.
 11. Process according to claim 10, wherein the rare-earthmetals content of said motor fuel is between 50 and 150 ppm.
 12. Processaccording to claim 1, wherein the temperature at which the gases arefiltered is between 200 ° and 350° C.
 13. Process according to claim 1,wherein said rare-earth metal is cerium or a cerium compound. 14.Process according to claim 1, wherein the surface area of the filter isselected so as to maintain the pressure drop in said filter at a valueat most equal to 0.3 bar.
 15. Process according to claim 1, wherein thetemperature of the gases in contact with the filter is selected so as tomaintain the pressure drop in said filter at a value at most equal to0.3 bar and advantageously to 0.2 bar.
 16. Process according to claim 1,wherein the content of rare-earth metals in the motor fuel is selectedso as to maintain the pressure drop in said filter at a value at mostequal to 0.3 bar.
 17. Process according to claim 1, wherein said rareearth metal contains lanthanum.
 18. Process according to claim 17,wherein said lanthanum is added to the motor fuel in the form of a saltwhich is stable in said motor fuel.
 19. Process according to claim 17,wherein the compound containing lanthanum is directly introduced intothe engine.
 20. Process according to claim 17, wherein the lanthanumcontent of the motor fuel is between 10 and 1000 ppm.
 21. Processaccording to claim 1, wherein the sol and/or the salt are introduced soas to form crystallite aggregates in which the largest size is between20 and 10,000 angstroms (2 and 1000 nm) and in which the size of thecrystallites is between 20 and 250 angstroms (2 and 25 nm).
 22. Processaccording to claim 17, wherein said lanthanum is introduced into theengine via air.
 23. Process according to claim 17, wherein saidlanthanum is added to the motor fuel in the form of a sol which isstable in said motor fuel.
 24. Process according to claim 1, wherein therare-earth metal or mixture of rare-earth metals is present at aconcentration effective to provide a back pressure of from 100 to 400millibar.
 25. Process according to claim 1, wherein the rare-earth metalor mixture of rare-earth metals is cerium and lanthanum alone or incombination with other rare earth metals.